A motor-driven screwdriver needs to be capable of tightening a screw with an appropriate rotational driving force because screw tightening with an excessive rotational driving force may damage the screw itself, or a member into which the screw is driven, or the motor-driven screwdriver itself. The motor-driven screwdriver is also used to loosen a tightened screw, and for this purpose, it is usually necessary to apply to the tightened screw a larger rotational driving force than that applied to tighten the screw.
FIG. 9 is a cross-sectional view of a rotational driving force transmission device 1 of a motor-driven screwdriver developed to meet the above-described technical demand, as seen toward the rear end of the motor-driven screwdriver opposite to the front end thereof provided with a screw bit. Accordingly, in the figure, counterclockwise rotation is forward rotation to tighten a screw, and clockwise rotation is backward rotation to loosen a screw.
The rotational driving force transmission device 1 has a rotational driving shaft 2 driven to rotate upon receiving rotational driving force from a driving motor, a circular cylindrical rotation output member 3 rotatable about the rotation center axis of the rotational driving shaft 2, and balls 4 held in the rotation output member 3 movably in the radial direction of the rotation output member 3 and subjected to radially inward urging force shown by the arrows 5. Rotational driving force from the rotational driving shaft 2 is transmitted to the rotation output member 3 through the balls 4, but when the rotational driving force exceeds a predetermined value, the balls 4 are pushed radially outward against the urging force 5, so that the rotational driving shaft 2 idles with respect to the rotation output member 3, thereby preventing a rotational driving force exceeding the predetermined value from being transmitted to the rotation output member 3. In addition, the rotational driving shaft 2 is shaped as shown in FIG. 9, thereby allowing the radial position for engagement of the rotational driving shaft 2 with each ball 4 to differ between backward rotation and forward rotation such that the rotational driving shaft 2 engages the ball 4 at a radially inner position during forward rotation than during backward rotation. Consequently, the proportion of the radially outward component of the force transmitted from the rotational driving shaft 2 to the balls 4 is smaller during backward rotation than during forward rotation, so that the rotational driving force required to move the balls 4 radially outward against the urging force 5 is larger during backward rotation. Accordingly, it is possible to transmit a larger rotational driving force when the rotation output member 3 is rotated backward to loosen a screw than when the rotation output member 3 is rotated forward to tighten a screw (Patent Literature 1).
FIGS. 10 and 11 show another rotational driving force transmission device 6. The rotational driving force transmission device 6 has a rotation input member 7 driven to rotate upon receiving rotational driving force from a driving motor, driving rollers 9 disposed in roller retaining portions 8, respectively, of the rotation input member 7, a circular cylindrical rotation output member 10 rotatable about the rotation center axis of the rotation input member 7, and driven balls 11 radially movably held by the rotation output member 10. The rotation output member 10 has a screwdriver bit (not shown) attached thereto. When the rotation input member 7 rotates forward (counterclockwise as seen in the figures), as shown in FIG. 10, the driving rollers 9 engage first retaining portions 8-1 of the roller retaining portions 8, respectively, and, in this state, engage the driven balls 11, respectively, to transmit rotational driving force to the rotation output member 10. When the rotation input member 7 rotates backward (clockwise as seen in the figures), as shown in FIG. 11, the driving rollers 9 engage second retaining portions 8-2 of the roller retaining portions 8, respectively, and, in this state, engage the driven balls 11, respectively, to transmit rotational driving force to the rotation output member 10. The driven balls 11 are urged toward the inside of the rotation output member, and when a force exceeding a predetermined value is applied thereto through the driving rollers 9, the driven balls 11 move outward, thereby allowing the rotation input member 7 to idle. The first retaining portion 8-1 and second retaining portion 8-2 of each roller retaining portion 8 are different in shape from each other as shown in FIGS. 10 and 11. The difference in shape allows the position for engagement of each driving roller 9 with the associated driven ball 11 to differ between forward and backward rotation such that the driving roller 9 engages the driven ball 11 at a position more away from the rotation center axis during backward rotation than during forward rotation, as in the case of the above-described example shown in FIG. 9. Accordingly, it is possible to transmit a larger rotational driving force during backward rotation than during forward rotation (Patent Literature 2).